First you have to be sure all the milk is cleaned from the carpet/upholstery. You can rent a steam cleaner at the grocery store if you don' t have one, or cannot borrow one. (Or, you could take the truck for a detailing.) You may need to go over it twice!

Once you know that the source of the odor is gone, you can deal with the residual odor with any of the methods people have mentioned in other posts. My personal favorite is coffee grounds, but kitty litter, baking soda, citrus, enzymatic (pet odor) eliminators, or the like caan work. Close up the truck, put the odor absorber/eliminator inside, and let it sit for a day or two.

I am sure by now you have scrubbed and cleaned the best you can. With everything you can think of, but use the cheapest shaving cream you can find. Yeap, you heard me correctly - shaving cream. Scrub it into the carpet really good with a clean cloth. DO NOT wet the carpet first.

Just put the shaving cream on the carpet, and start scrubbing. Then after a few minutes of scrubbing, walk away for a little while. Let it dry for a little while. Vacuum, then sprinkle the area with baking soda. Let that sit for a while, and then vacuum that up. The longer the baking soda sits, the better. It really soaks up all that odor really well, as does the shaving cream. Shaving cream is great on so many stains on carpet in your home also. I did this when my grandson threw up in our truck, and it worked really well.